Howard Medium Power Substation - Manual (Page 11)

Medium Power Substation Transformers
Document 2.4.06, Revision 4
June 2013
11
When internal work is being done on the transformer,
dry air should be used to ventilate the inside of the tank.
Dry air can be obtained in high-pressure cylinders or
from desiccant-type air-drying equipment. When using
dry air, the following precautions should be observed.
1. The temperature of dry air entering the
transformer should equal to or greater than the
temperature of the transformer fluid and at least
10°F higher than the dew point of the ambient
air.
2. The dry air flow should be directed so as to
continuously purge the transformer gas space.
3. The dew point in the tank gas space should
never exceed 20°F.
While the tank is open, the following tests should be
conducted.
1. A ratio test should be made on all windings
and on each tap position. Contact the Howard
Industries Substation Transformer Division if any
measurement is off-ratio by more than 0.5%.
2. Insulation resistance of each winding to all
other windings and from each winding to ground
should be measured. (Coil windings must be
completely covered with fluid.) Record the
temperature of the fluid. These readings should
be comparable with factory measurements.
3. Disconnect the core ground connection, when
accessible, on core-form transformers and
measure the resistance from the core to the
tank or end frames, using a 1000-volt Megger
test set. The resistance should exceed 100
megohms if the core is not covered with fluid or
200 megohms if the core is covered with fluid.
When the internal inspection is complete, reseal the
tank and refill with fluid (if any was removed prior to
inspection). The gas space should be filled with dry
air or dry nitrogen. Contact the Howard Substation
Transformer Division if any internal damage is found
during the internal inspection.
Installation of Detachable Radiators
When radiators are shipped detached, they should be
installed according to the instructions below.
1. verify that all valves on the tank are in the
closed position.
2. Remove shipping plates from the valves on the
tank and from the radiator flanges. Take care
not to damage gaskets.
3. Inspect for moisture and contamination
inside the radiator headers. Contact the
Howard Substation Transformer Division if any
contamination is found.
4. Clean all mating surfaces on the tank valves
and radiator flanges and apply a small amount
of petroleum jelly to the gaskets on the tank
valve flanges to hold them in place during
installation of the radiators.
5. lift the radiators into position using the lifting
eyes on top of the radiators and align holes
in the radiator flanges with holes in the valve
flanges. If fans are provided, attach the
radiators having fans first, then attach the other
radiators.
6. Insert bolts in the top and bottom flanges, with
bolt heads adjacent to the transformer tank.
7. Tighten bolts evenly, alternating across corners,
until metal-to-metal tightness is achieved at all
four corners of each flange.